As the critical dimension of metal oxide semiconductor field effect transistor (MOSFET) devices continues to shrink, the short channel effect becomes a critical issue. Fin field effect transistor (FinFET) devices have a good gate control capability to effectively suppress the short channel effect. FinFET devices also reduce random dopant fluctuation to improve the stability of the devices. Thus, FinFET devices are generally used in the design of small-sized semiconductor elements.
The work function plays an important role in the regulation of the threshold voltage of a FinFET device. In high-k dielectric layer and metal gate (HKMG) gate-last processes of FinFET devices, a compound containing aluminum (Al) is generally used as an N-type work function metal layer, however, the present inventor has discovered that N-type work function metal layers can be easily oxidized, resulting in the change of the work function of an NMOS device, thereby affecting its threshold voltage.
Therefore, there is a need for an improved gate stack structure and manufacturing methods thereof to stabilize the work function of an NMOS device.